This invention relates generally to techniques and apparatus for image sensing, and more particularly to a method and apparatus for obtaining a digital measurement of a photo-current.
A typical optical detector, such a charge-coupled device (CCD), consists of an array of pixels each with its own pn junction photodiodes. Each photodiode has a capacitance associated with it, and when light falls on the detector the resulting photo-current charges the capacitance. The accumulated charge is thus the time integral of the light intensity falling on each pixel. The CCD periodically and sequentially switches the charge to a video line, resulting in a series of pulses, which can be converted into a voltage signal representing the light pattern incident on the array. If the integration time is too long, the device will saturate. If the integration time is too short, the voltage signal will be lost in the noise of the device. When a detector is reset the capacitor is discharged the voltage across the capacitor does not go to zero because of residual electrical noise in the circuit. The noise is referred to as reset noise. For example, a root-mean-square (rms) level of about 100 electrons might remain in the output sense capacitor after each pixel""s charge packet is read. The reset noise increases in proportion to temperature, so the problem is more of a concern in devices that need to operate at higher temperatures.
One approach to reducing reset noise is the use of analog differential measurements through a double sampling technique. This approach first mirrors the initial charge offset in the output sense capacitor onto another capacitor and then reads the accumulated signal charge at the predetermined integration time. A differential measurement is performed on these two analog values either on a per-column basis or a on a per-pixel basis. Column-based calculations require twice the bandwidth while pixel-based differential measurements are sensitive to higher temperatures since the initial reading is fed as an output to a differential amplifier. Technology scaling produces additional concerns, as differential measurements become more sensitive to temperature variations and lower biasing levels make reset operations harder to perform.
In light of the foregoing discussion, it can be seen that there is a need in the art to have pixel sensors and image sensor arrays that are more robust in the presence of reset noise.